This invention relates to thermoformed multilayer structures having at least one layer of a blend of an amorphous polyamide polymer having a Tg greater than about 120.degree. C. with an aliphatic polyamide polymer and at least one layer of a structural thermoplastic polymer.
Containers and films with good gas barrier properties are needed for the packaging of perishable items such as foods, drinks, pharmaceuticals and cosmetics. Amorphous polyamides have been shown to possess both excellent oxygen barrier properties and optical properties and are therefore desired to be used in conjunction with other, often less expensive, structural materials, e.g., as in a multilayer film or container. Containers and films containing such amorphous polyamides can be made by thermoforming processes. For the purpose of this invention, thermoforming processes include any process for forming a shaped article (e.g., a film or a container) which is performed in a process which (a) is distinct from the initial melt processing step and (b) which is performed at a temperature which is elevated but substantially lower (e.g., by at least about 30.degree. C.) than that required in the melt processing step. These thermoforming processes are performed on a semi-finished shaped article (often called a "preform") which was cast or molded from a molten polymer. Thus, for example, extrusion of a film would not be a thermoforming process according to this invention because it is a melt processing step; vacuum-forming the film to prepare a container would be a thermoforming process. Examples of thermoforming processes include thermoforming as the term is commonly used,(but excluding melt phase thermoforming), vacuum-forming, solid phase pressure forming, co-injection blow-molding, co-injection stretch blow-molding, tube extrusion followed by stretching, scrapless forming, forging and tubular or flat sheet oriented film processes. Examples of articles that can be prepared using thermoforming processes are films and containers such as bottles, jars, cans, bowls, trays, dishes and pouches.
A problem that arises in producing thermoformed articles incorporating amorphous polyamides is that the temperature required for thermoforming many of such polyamides (e.g , those with a glass transition temperature exceeding about 120.degree. C.) is substantially higher than that of many of the structural materials desired to be used in conjunction with the amorphous polyamide and/or higher than that which available thermoforming processes and equipment can accommodate. Thus, for example, when one attempts to form a coinjection stretch blow-molded two-layer container from an amorphous polyamide (e.g., a copolymer of hexamethyldiamine with isophthalic and terephthalic acids) and polyethylene terephthalate (PET) at the forming temperature for the PET, an unsatisfactory result is likely to be achieved. Stretch blow-molding at the stretching temperature for the PET will produce a container with a non-uniform layer of amorphous polyamide because the polyamide is not stretchable at the stretching temperature for the PET. On the other hand, attempts to stretch blow mold at the stretching temperature for the amorphous polyamide will lead to a defective container because PET starts to crysatallize if it is heated above 100.degree.-105.degree. C., and the crystallization leads to reduced optical clarity. It is therefore an object of this invention to disclose a thermoformed multilayer structure having the optical and barrier properties provided by amorphous polyamides but capable of being thermoformed at temperatures less than the temperature required for thermoforming most amorphous polyamides.